(1) Field of the Invention
The present invention relates to a mechanical filter and specifically to a mechanical filter of the type of differential coupling.
(2) Prior Art
A mechanical filter is commonly used as a band-pass filter and deals with transformation of electrical signals with ultrasonic waves as a medium, and vice versa. This type of filter exhibits numerous advantages, such as having a high Q compared with electric elements L and C, while maintaining or increasing stability and enabling the device itself to be of a small construction. Because of these advantages, the mechanical filter has been used as a channel filter for channel translating equipment or as a filter for navigation receivers or for automatic train control systems.
A mechanical filter of the simplest form consists of a pair of mechanical vibrators that are coupled together by means of a single coupler. Even mechanical filters having a complicated construction are obtained by combining mechanical filters of the above-mentioned basic form.
Many of these mechanical filters employ a differential coupler which is effective when the mechanical filters are used as narrow-band filters. The reasons are discussed below.
In general, the equivalent stiffness of a coupler required for a narrow-band mechanical filter decreases with the decrease in the band width of the filter. The equivalent stiffness, on the other hand, depends upon the shape and size of the coupler and the constant of the material. For instance, a small equivalent stiffness can be obtained if the propagation velocity and the diameter of the coupler are decreased and the length of the coupler is increased. Because of the requirements for the mechanical strength of mechanical filters, however, a limitation is imposed on the length of the coupler. Furthermore, a limitation is imposed on the propagation velocity of the coupler, although it can be varied to some extent by varying the temperature of the heat treatment for the coupler.
In view of the above, the only way to produce a small equivalent stiffness when the coupling position of the coupler is constant, is to reduce the diameter of the coupler. For example, if it is intended to construct a mechanical filter using a single coupler, the mechanical strength of the filter inevitably decreases. On the other hand, when a mechanical filter is to be constructed using two couplers based upon the aforementioned differential coupling, such a mechanical filter can be obtained with the two couplers having relatively large diameters even though their equivalent stiffness may be considerably small. In other words, it is possible to easily obtain a narrow-band mechanical filter having increased mechanical strength. The second advantage is with regard to the design and manufacturing. Generally, it is desired that the mechanical filters be constructed using conventional standardized couplers from the viewpoint of economy and ease of manufacturing. However, if a mechanical filter is produced using a single coupler, the equivalent stiffness of the coupler varies depending upon the kind of the standardized product, and this variation limits freedom of the design. On the other hand, when two couplers are employed, freedom of the design of the equivalent stiffness is not limited, depending upon the combination of the two couplers.
The foregoing mentioned the principal advantages of using differential couplers to manufacture mechanical filters. A variety of methods have heretofore been proposed to obtain a differential coupling of practical mechanical filters.
A first method consists of connecting a coupler to a pair of mechanical vibrators, which vibrate in the same mode and at the order of the same number, at portions at which the mechanical vibrators vibrate in directions which differ by 180.degree. from each other. A second method consists of connecting a coupler to a pair of vibrators, which vibrate in different vibration modes and at orders of different numbers, at portions at which the mechanical vibrators vibrate in directions which differ by 180.degree. from each other.
The above two methods have advantages and disadvantages. Referring to the first method, for example, the two mechanical vibrators undergo vibration in the same mode and at the order of the same number. Therefore, the two mechanical vibrators must have the same size and shape. Accordingly, when the mechanical filter is manufactured, mechanical vibrators having the same size and same shape may be used. However, the couplers must be connected in a tilted direction or in a diagonal direction with respect to the mechanical vibrators, and this presents a problem from the standpoint of manufacturing the mechanical filter, and also a high accuracy of the mechanical filter construction can not be expected.
Referring to the second method, the two couplers are connected to a pair of mechanical vibrators at right angles therewith, enabling the mechanical filter to be easily manufactured while maintaining increased precision. However, since the two mechanical vibrators vibrate at different modes, the two vibrators must be formed in different shapes. Besides, since the two mechanical vibrators vibrate at different modes as mentioned above, they must be made of different materials. Moreover, in many cases, the two couplers are connected to the upper and lower surfaces of the mechanical vibrators which requires an increased number of manufacturing steps.